An MR-PET scanner has the potential of acquiring anatomical, functional and gross metabolic tissue status through MRI/MRS and metabolic activity through PET. The ultimate goal of a hybrid scanner is to yield temporal and spatial coregistration of MR and PET data. The objective of this research is to demonstrate the feasibility of developing a hybrid MR-PET scanner that performs MR and PET tomography on bioreactors or small animals. To achieve this goal a section of a magnetic field tolerant PET ring will function in the center of a 5.0 T/40cm MR magnet. This is a continuation of the work done through Small Grant for Innovative Technology, NIH/I-R03-RR07O42-01, that experimentally showed positron range squeezing in a magnetic field. This proposal should lay the foundation for engineering a hybrid MR-PET scanner that simultaneously performs MR and PET tomography. An advantage of conducting a PET experiment in a magnetic field is that intrinsic in-plane spatial resolution improves. This is important for small diameter PET rings whose spatial resolution may exceed the intrinsic spatial resolution of a PET image due to positron range blurring. Thus, especially for the case of high energy positron emitters (e.g., 11-C, 15- O, 68-Ga, 82-Rb), the possibility of acquiring high resolution in-plane images (<2 mm/pixel) from animals should be possible. Positron range in a magnetic field is anisotropic. A positron with a momentum component transverse to the magnetic field limits the positron's range whereas a positron whose momentum is co-linear with the field is unaffected. Integrating MR and PET hardware into the same scanner is not trivial. PET ring components must be tolerant to time dependent and time independent magnetic fields, that is, pulsed gradients, radiofrequency fields (B1) and a strong static magnetic field (B-0), respectively. Conversely, the PET ring can degrade MR imaging or spectroscopy by distorting B-0 through susceptibility and eddy current interactions, coupling to the RF coil and injecting noise into the NMR system. Although MR and PET interactions are troublesome we can minimize these effects with proper engineering. As a first step toward realizing the development of such a multimodality device we have chosen to focus on evaluating the interaction of MR system components on PET system components and vice versa. This research will culminate in obtaining an MR-PET image of a functioning bioreactor.